In many automated mechanical operations it is often necessary to sense the distance to a surface or to maintain a fixed distance between two surfaces. In some applications this distance or gap that is to be sensed or maintained is very small, in the order of microns or millionths of a meter. One application where these gaps need to be detected or maintained is in lithographic systems such as those used in semiconductor manufacturing.
Typically, air gauges have been used applying the principles of a pressure difference between two balanced sides or legs. The deflection of a flexible membrane between the two legs is used to measure or maintain a gap or distance between two surfaces. One surface being a probe emitting the air and the other surface being the surface the distance from which is to be measured.
While these air gauges are acceptable for most applications, in some applications their performance is not adequate. For example, because the air gauges operate on a pressure difference the pressure change must be integrated over the volume of the air conduits resulting in a long response time. This response time, or frequency response, is in the order of 200-300 msec. Additionally, these air gauges operate at relatively high pressures resulting in high airflow increasing the liklihood of contamination. As technology advances, and in particular the lithographic arts as applied to semiconductor manufacturing, these response times limit system performance. Therefore, there exists a need to improve the response time of air gauges generally, and more specifically as applied to lithographic systems used in the manufacture of semiconductors.